HOST-SELECTION 287 



slight structural differences, or differences in size or colour, are 

 involved, in other cases not. For many years experimental studies 

 of host-preferences of this kind, and of their possible repetition in 

 succeeding generations, was lacking. It is only comparatively 

 recently that the problem has received critical investigation, but 

 in all cases larger scale and more prolonged studies are still a 

 desideratum. The whole subject is not only of importance in 

 relation to the general question of the origin of species, but also 

 from the standpoint of applied entomology. 



It is necessary here to refer briefly to the instance of host-selection 

 specifically stressed by Hopkins. His example concerns the moun- 

 tain pine beetle {Dendroctonus monticola), which attacks various 

 species of pines. According to him, in this insect, when it becomes 

 established in one species of pine through many generations, the 

 beetles upon emergence show a marked preference for that 

 species, and will not attack any other. Hopkins applied this 

 observation to control measures, and held the view that each 

 such " strain " can be dealt with as if it were a separate species. 

 In 1922 Craighead published the results of a number of years' 

 experiments with about a dozen species of Cerambycidse, which 

 afforded support to Hopkins' general contention. He found that, 

 in practically all the species studied, the adults exhibited marked 

 predilection for the host-species in which their larval existence 

 was passed, provided they were not deterred by such factors as 

 unfavourable conditions or the scarcity of their host. Con- 

 tinued breeding appeared to intensify the preference and, in some 

 species, transference to another species of host for only a year 

 was said to be sufficient to determine a preference for this host 

 by the resulting adult beetles. In 1927 Larson discussed the 

 host-selection principle very fully and tested it with reference to 

 the cowpea weevil (Bruchus quadrirnaculatus). His results were 

 not in accordance with those of Craighead, since he concluded that 

 the insect in question showed no marked preference for the host 

 in which it had been bred, and that continued breeding in any 

 one specific host did not appear to intensify preference for that 

 host. In 1928 Thompson and Parker examined the question 

 with regard to the corn borer (Pyrausta nuhilalis). This insect 



